Alkanesulfonic acids, particularly methanesulfonic acid (MSA), are typically prepared by reaction of a thiol or disulfide with a strong aqueous oxidant. A common oxidant for this chemistry is aqueous chlorine, with alternative oxidants including nitric acid and hydrogen peroxide/hydrogen halide. The alkanesulfonic acid products from these oxidation reactions are typically in an aqueous form. The purified commercial products are then sold as aqueous acids or anhydrous acids, the latter being typically obtained by distillation of the water from the sulfonic acid.
Processes for the distillation of water from aqueous methanesulfonic acid to produce anhydrous sulfonic acids, particularly anhydrous methanesulfonic acid, require high temperatures (120-190° C.) and reduced pressures (5-66 mbar) to effect the removal of the water and to obtain an anhydrous product. At these elevated temperatures, anhydrous or near-anhydrous alkanesulfonic acids are known to decompose to form alkyl alkanesulfonate esters, presumably via formation of the sulfonyl anhydride and subsequent decomposition to the alkyl alkanesulfonate ester and SO2. In the case of methanesulfonic acid, the adventitiously formed methyl methanesulfonate is known to possess toxic or mutagenic properties, and its presence is therefore undesirable. Other alkyl esters of methanesulfonic acid have also been shown to exhibit high toxicity and mutagenic properties.
Alkanesulfonic acids are commonly used as catalysts for the esterification of weaker acids with alkanols, and in the preparation of ethers from alkanols. These reactions are normally driven to completion by removal of the water of condensation using distillation or liquid/liquid phase separation. In these reactions, esterification of the sulfonic acid catalyst can occur and thereby consume some of the sulfonic acid, which could otherwise be recycled for further use as a catalyst. Thus in such systems, as well as in the manufacture of alkanesulfonic acids, it would be desirable to provide ways of converting such sulfonate esters back to the acids, or to permit their facile removal.